Radiant electric heater

ABSTRACT

A radiant electric heater comprises a dish-shaped enclosure ( 1 ) formed of a first thermal insulation material and having a base ( 3 ) and a peripheral wall ( 5 ) and at least one upwardly-extending protrusion ( 7 ) formed in the base and defining a channel ( 9 ) in the base within the peripheral wall. A layer ( 11 ) of a second thermal insulation material, having greater thermal insulation properties than the first thermal insulation material, is provided in the channel. A radiant electric heating element ( 13 ) is supported on or in the layer of the second thermal insulation material.

This invention relates to a radiant electric heater, and in particular to a radiant electric heater which incorporates two different types of thermal insulating material.

Radiant electric heaters, for example for glass ceramic cooktops, operate at high temperatures, in the region of 1100 degrees Celsius. Maximum cooking performance is therefore achieved by using a thermal insulation material which has optimum characteristics in terms of thermal conductivity, infra-red reflectivity, electrical insulation and mechanical strength.

However, such thermal insulation materials are costly and it is therefore desirable to use a minimum amount of such insulation in order to minimise overall cost.

It is known, for example, from U.S. Pat. No. 5,532,458 to provide a radiant electric heater for use with a glass ceramic cooktop which comprises a dish-like enclosure of highly porous inorganic material, such as vermiculite, together with a binder to provide a relatively low-performance thermal insulation material within which is provided a layer of a high-performance microporous thermal insulation material comprising a finely divided metal oxide, such as pyrogenic silica, optionally with an opacifier and/or an inorganic binder. A circular opening is preferably provided in the base of the vermiculite enclosure which in practice reduces the amount of the vermiculite insulation material that is required.

A disadvantage of such a construction is that, while it may reduce the amount of one of the components, the component concerned is the relatively inexpensive vermiculite thermal insulating material and not the relatively expensive microporous thermal insulation material.

It is therefore an object of the present invention to provide a radiant electric heater which reduces the amount of the relatively expensive microporous thermal insulation material required.

According to the present invention there is provided a radiant electric heater comprising:

-   -   a dish-shaped enclosure formed of a first thermal insulation         material and having a base and a peripheral wall and at least         one upwardly-extending protrusion formed in the base and         defining a channel in the base within the peripheral wall;     -   a layer of a second thermal insulation material, having greater         thermal insulation properties than the first thermal insulation         material, provided in the channel; and     -   a radiant electric heating element supported relative to (on or         in) the layer of the second thermal insulation material.

The upper level of the upwardly-extending protrusion may not be higher than the upper level of the peripheral wall and is preferably somewhat lower than the upper level of the peripheral wall.

The layer of the second thermal insulation material may be substantially annular in configuration, extending between the upwardly-extending protrusion and the peripheral wall. Alternatively, the layer of the second thermal insulation material may be in the form of a narrow track which corresponds to the course of the heating element.

The first thermal insulation material may comprise a highly porous inorganic material which incorporates a high proportion of silicon dioxide. The first thermal insulation material may not be microporous.

The first thermal insulation material may be selected from an expanded sheet silicate, such as vermiculite or mica, highly porous volcanic material, such as perlite or pumice, siliceous fossil earth, such as kieselguhr, or plant ash, such as rice ash or maize ash, or cementitious materials such as portland cement and quicklime, and mixtures thereof.

Ideally, the highly porous inorganic material is vermiculite.

The first thermal insulation material may include a binder, for example in an amount in the range from 0.01 to 40 percent by weight, preferably in the range from 10 to 30 percent by weight. The binder may be selected from an aqueous phosphate solution, such as monoaluminium phosphate, a silicophosphate, an alkali metal water glass, or a silica sol, and mixtures thereof.

The second thermal insulation material may be compacted microporous thermal insulation.

The microporous thermal insulation material may comprise:

-   -   30 to 100 weight percent microporous, finely divided metal         oxide;     -   0 to 50 weight percent opacifier;     -   0 to 50 weight percent reinforcing fibre;     -   0-15 weight percent inorganic binder.

The finely divided metal oxide may be silica and/or alumina. The finely divided metal oxide may preferably be present in a range from 50 to 90 percent by weight.

The opacifier may be selected from titanium dioxide, such as rutile, ilmenite, silicon carbide, iron oxide, chromium dioxide, zirconium oxide, manganese dioxide, zirconium silicate and mixtures thereof. The opacifier may preferably be present in a range from 20 to 50 percent by weight.

The reinforcing fibre may be selected from glass wool, glass fibres, aluminosilicate fibres, rock wool, ceramic fibres, such as of alumina and/or silica, and mixtures thereof. The reinforcing fibre may preferably be present in a range from 5 to 20 percent by weight.

The inorganic binder may preferably be present in a range from 0 to 2 percent by weight.

The heating element may comprise a corrugated ribbon heating element partially embedded edgewise in the layer of the second thermal insulation material. Alternatively, the heating element may comprise a coiled heating element secured to the layer of the second thermal insulation material, for example by means of staples.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which:

FIG. 1 is a diagrammatic cross-sectional view through one embodiment of a radiant electric heater according to the present invention; and

FIG. 2 is a view of part of another embodiment of a radiant electric heater according to the present invention.

The radiant electric heater shown in FIG. 1 comprises a dish-shaped enclosure 1 which includes a base portion 3 and a peripheral wall 5. The base portion 3 is continuous in that it has no apertures, however, it is not necessarily planar on its lower side, while on its upper side it has at least one upwardly-extending protrusion 7. In the illustrated example, the upwardly-extending protrusion is arranged substantially centrally within the peripheral wall 5 and is generally circular in configuration, although as an alternative in a rectangular heater the protrusion 7 could also be substantially rectangular. The upper level of the upwardly-extending protrusion 7 is not higher than the upper level of the peripheral wall 5 and in general is somewhat lower than the upper level of the peripheral wall. Consequently there is formed in the base portion 3 within the peripheral wall 5 a channel or trough 9 which in the illustrated embodiment is generally annular in configuration.

The dish-shaped enclosure 1 is made of an inorganic material which is highly porous and incorporates a high proportion of silicon dioxide, but which is not microporous. Suitable materials include, either alone or in combination, expanded sheet silicates, such as vermiculite and mica, highly porous volcanic materials, such as perlite and pumice, siliceous fossil earths, such as kieselguhr, and plant ashes, such as rice ash and maize ash, or cementitious materials such as portland cement or quicklime. Most preferable is expanded vermiculite without any additional highly porous inorganic material. To manufacture the dish-shaped enclosure, a binder is mixed with the highly porous inorganic material in an amount in the range from 0.01 to 40 percent by weight, preferably in the range from 10 to 30 percent by weight. The binder may be, for example, one or more of an aqueous phosphate solution, such as monoaluminium phosphate, a silicophosphate, an alkali metal water glass, or silica sol. Additionally, a foaming agent, such as powdered aluminium, sodium bicarbonate and/or flour can be used.

A microporous thermal insulation material is compacted into the trough 9 to provide an annular layer 11 of compacted microporous thermal insulation. The thickness of the layer 11 is such that the upwardly-extending protrusion 7 and the peripheral wall 5 are at a higher level than the layer 11.

The microporous thermal insulation material is based on a microporous, finely divided metal oxide, for example of silica and/or alumina in an amount in the range from 30 to 100 percent by weight, preferably, 50 to 90 percent by weight. To this is added an opacifier in an amount from 0 to 50 percent by weight, preferably, 20 to 50 percent by weight, a reinforcing fibre in an amount from 0 to 50 percent by weight, preferably from 5 to 20 percent by weight, and an inorganic binder in an amount from 0 to 15 percent by weight, preferably from 0 to 2 percent by weight.

The finely divided metal oxide has a specific surface area, measured by the BET method, in the range from 50 to 700 m²/g, preferably 70 to 400 m²/g and ideally substantially 200 m²/g. The finely divided metal oxide may be made by a pyrogenic process, by precipitation or may be an aerogel.

The infra-red opacifier may be, for example, one or more of titanium dioxide (such as rutile), ilmenite, silicon carbide, iron oxide, chromium dioxide, zirconium oxide, manganese dioxide or zirconium silicate.

The reinforcing fibre may be one or more of glass wool, glass fibres, aluminosilicate fibres, rock wool, or ceramic fibres, for example of alumina and/or silica.

A heating element 13 is mounted relative to the microporous layer 11. As illustrated, a corrugated ribbon heating element is partially embedded edgewise in the microporous layer. Alternatively, a coiled heating element may be secured to the microporous layer, for example with metal staples. Other forms of heating element are also possible.

The microporous layer 11 provides a layer of high-performance thermal insulation material in the region of the heating element to minimise conduction of heat into the heater and to reflect incident radiation from the surface of the layer 11 as illustrated diagrammatically in FIG. 1. The dish-shaped enclosure 1 provides a relatively inexpensive support for the layer 11 of microporous thermal insulation material, while the upwardly-extending protrusion 7 occupies a region within the peripheral wall 5 where there is no heating element 13 and therefore reduces the extent of the microporous layer 11 and consequently reduces the overall cost of the radiant electric heater.

The radiant electric heater illustrated in relation to FIG. 2 develops the concept of FIG. 1 and the same references are used to denote the same or similar components. Essentially, in place of the single annular layer 11 of microporous thermal insulation material in FIG. 1, there is a narrow track 15 of microporous thermal insulation material which corresponds to the course of the heating element 13, with a number of upwardly extending protrusions 17 of non-microporous thermal insulation material between each of the tracks 15, each protrusion extending to a height above the surface of the tracks 15. The tracks may each be independent with a separate heating element for each track, or they may be arcuate but interconnected in a radial direction with one or more heating elements passing from the periphery towards the centre of the heater and back again by way of the radial interconnections, or a single spiral track may be provided which conducts one or more heating elements from the periphery of the heater towards the centre and back again along the spiral track. 

1. A radiant electric heater comprising: a dish-shaped enclosure (1) formed of a first thermal insulation material and having a base (3) and a peripheral wall (5) and at least one upwardly-extending protrusion (7) formed in the base and defining a channel (9) in the base within the peripheral wall; a layer (11) of a second thermal insulation material, having greater thermal insulation properties than the first thermal insulation material, provided in the channel; and a radiant electric heating element (13) supported relative to the layer of the second thermal insulation material.
 2. A heater as claimed in claim 1, wherein the upper level of the upwardly-extending protrusion (7) is not higher than the upper level of the peripheral wall (5).
 3. A heater as claimed in claim 2, wherein the upper level of the upwardly-extending protrusion (7) is lower than the upper level of the peripheral wall (5).
 4. A heater as claimed in claim 1, wherein the layer (11) of the second thermal insulation material is substantially annular in configuration, extending between the upwardly-extending protrusion (7) and the peripheral wall (5).
 5. A heater as claimed in claim 1, wherein the layer (11) of the second thermal insulation material is in the form of a narrow track which corresponds to the course of the heating element (13).
 6. A heater as claimed in claim 1, wherein the first thermal insulation material comprises a highly porous inorganic material which incorporates a high proportion of silicon dioxide.
 7. A heater as claimed in claim 6, wherein the first thermal insulation material is selected from an expanded sheet silicate, highly porous volcanic material, siliceous fossil earth, or plant ash, or cementitious materials, and mixtures thereof.
 8. A heater as claimed in claim 7, wherein the highly porous inorganic material is vermiculite.
 9. A heater as claimed in claim 1, wherein the first thermal insulation material includes a binder.
 10. A heater as claimed in claim 9, wherein the binder is present in an amount in the range from 0.01 to 40 percent by weight.
 11. A heater as claimed in claim 10, wherein the binder is present in an amount in the range from 10 to 30 percent by weight.
 12. A heater as claimed in claim 9, wherein the binder is selected from an aqueous phosphate solution, a silicophosphate, an alkali metal water glass, or a silica sol, and mixtures thereof
 13. A heater as claimed in claim 1, wherein the second thermal insulation material is compacted microporous thermal insulation.
 14. A heater as claimed in claim 13, wherein the microporous thermal insulation material comprises: 30 to 100 weight percent microporous, finely divided metal oxide; 0 to 50 weight percent opacifier; 0 to 50 weight percent reinforcing fibre; 0-15 weight percent inorganic binder.
 15. A heater as claimed in claim 14, wherein the finely divided metal oxide is silica and/or alumina.
 16. A heater as claimed in claim 14, wherein the finely divided metal oxide is present in a range from 50 to 90 percent by weight.
 17. A heater as claimed in claim 14, wherein the opacifier is selected from titanium dioxide, ilmenite, silicon carbide, iron oxide, chromium dioxide, zirconium oxide, manganese dioxide, zirconium silicate and mixtures thereof.
 18. A heater as claimed in claim 14, wherein the opacifier is present in a range from 20 to 50 percent by weight.
 19. A heater as claimed in claim 14, wherein the reinforcing fibre is selected from glass wool, glass fibres, aluminosilicate fibres, rock wool, ceramic fibres, and mixtures thereof.
 20. A heater as claimed in claim 14, wherein the reinforcing fibre is present in a range from 5 to 20 percent by weight.
 21. A heater as claimed in claim 14, wherein the inorganic binder is present in a range from 0 to 2 percent by weight.
 22. A heater as claimed in claim 1, wherein the heating element (13) comprises a corrugated ribbon heating element partially embedded edgewise in the layer (11) of the second thermal insulation material.
 23. A heater as claimed in claim 1, wherein the heating element (13) comprises a coiled heating element secured to the layer (11) of the second thermal insulation material.
 24. A heater as claimed in claim 23, wherein the coiled heating element (13) is secured to the layer (11) of the second thermal insulation material by means of staples. 